Intentional extreme defocus

GF staff, close your ears.

Has anyone though to try doing a really defocused score by intentionally flipping the lens upside down? Maybe? No?

I’m going to do one now, let’s see. I’ll use some scrap cork paper I had laying around, since it’s thin and picks up scoring so easily, it should let me see if there are real differences.

The result? Actually not much difference.

Ah well, it was worth a shot. Even if it’d worked, I’m not sure I would recommend doing extensive upside down ops, as the “wrong side” of your lens will be subject to much more fouling than usual.


An upside down lens will come into focus at some point. I think you are getting pretty close to the “upside-down focal point.”

I would suggest doing your upside down defocused score at the same focal height as your in-focus score. You should see a much more defocused line.

Hmm, I did?

On the left are two, one in focus, and one way out of focus. On the right are the same jobs repeated but with the lens upside down. Am I not understanding what you are saying?

Ahh I see, the red and green areas.

No reasonable conclusion that I can come up with for your top green line being in focus with an upside down lens. Time and time again we have seen how far off putting the lens in upside down will change the line characteristics.

Are you setting the material height to a value and then making the focal height of the job a value different than the material height?

Material height is 0.05. That is the true height of the cork.

I set the jobs up to a speed and power that was the same in both cases, and the power was very low. Cork doesn’t take much to get a dark line.

I then modified the focal height of one line to 0.05, and the focal height of the other to 0.5 in the custom settings. Standard defocus techniques.

I’m a sample size of one, so maybe someone else can attempt to repeat my work. It’s possible that higher power levels will yield more dramatic results.


Haven’t people damaged their lens from replacing it upside down? This is an interesting test, though. My assumption of the lens was that it would spread the beam more if it were upside down. Have you tried changing any other settings outside of focus, not to say that that could affect it, but maybe more power means a thicker beam.

Generally yes, this is why I chose cork, because it goes jet black at a mere hint of laser.

I figured that would show even the out of focus fringe of the beam better than most material, though it might require more power to really see a difference. Give it a whirl? I’m mid big job.

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Don’t take this with any definitiveness, but I think you’ll be fine. I can’t think of a logical reason for it to damage the lens itself. The beam is hitting the lens sooner, but I don’t think it has any appreciable change in beam size over that small distance between cup is up, or cup down that would cause it to hurt anything. Obviously not a glowforge recommended technique though. :slight_smile:


Here you go. Measurements are above surface of material. Markings should make it obvious which set is which. I always include the KB for scale, but for reference, the circles are .5". This is 1/8" BB from Rockler. 125/21 (default Maple Ply score)

I might repeat the 1/4" cut tests I did at different focus heights, but with the lens inverted.

On a related note, I often thought how hard would it be to just use a sharpie to put an arrow on the lens when you remove it, so I did. However, when I went to install it upside down for this test, I discovered, it’s virtually impossible to get the thing to balance on the lens tool when it’s upside down!! I had to hold it in place to prevent dropping it. I almost snaps into place when it’s on there correctly.

How people can install these incorrectly is beyond me. It’s not even worth “sharing” my idea.


Might help if I remove the masking! lol…


I discovered quite by chance that mine actually has an arrow engraved in the housing. :stuck_out_tongue_winking_eye:

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So I’d guess that this is a success in terms of getting a wider line on a score. Your result definitely shows some difference.

That’s cool to know.

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I just realized the numbers must be wrong. I couldn’t have set the defocused scores to (material thickness)+0.5. I checked, and it’s actually 0.5, sorry.

Works for this thread I guess.

Yeah I am not sure when that was done but all the new ones seem to have it now.

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I usually have the head off of the machine, so I just drop my lens in, pushing it in with the removal tool, so it would be quite easy to install it the wrong way if it was done the way I do it.